The present invention relates to a method for producing a filler material suitable for use in the plastics and concrete industries from fly ash. More specifically, this invention relates to a method for reducing particle flocculation, thereby allowing production of a fly ash-based filler material containing a narrow range of particle sizes.
Plastics have become the dominant material of our age, replacing wood, metal and ceramics. Plastic products almost never contain the pure polymer but are compounded materials containing plasticizers, antioxidants, biocides, UV blockers, fire and smoke retardants, coloring agents (stains and pigments) and mineral fillers. Substances previously used as mineral fillers include naturally occurring or precipitated calcium carbonate (CaCO3, primarily as calcite), aluminum trihydroxide (Al(OH)3 or ATH), calcium silicate (CaSiO3, as Wollastinite), carbon black, titanium dioxide (TiO2, rutile), ground fiberglass, ground glass, talc and fly ash. The total usage and value of fillers is difficult to estimate as much of the production of fillers and additives is tightly controlled. However, filler production is in the range of millions of tons with monetary value in the billions of dollars.
Mineral fillers have two major functions in the plastics industry. First, they may be utilized as material extenders to displace the resin or polymer and reduce the overall cost of the product. Second, they may be used as material modifiers to change the physical characteristics of the final product. Thermoplastic polymers filled with rigid inorganic particles display higher values of Young""s modulus, better thermal stability and lower wear under friction than unfilled polymers. However, certain prior art mineral fillers may also greatly reduce the polymer tensile strength, disadvantageously contributing to embrittlement.
Addition of other prior art fillers such as calcite or talc to plastics causes problems in the compounding step by entraining air. Also, these materials greatly increase the viscosity of the melt, making injection molding more difficult. The shape and texture of the siliceous glassy portion of fly ash provide certain superior and desirable attributes as a mineral filler for, e.g., plastics. The smooth round surface of fly ash reduces both air entrainment and melt viscosity compared to other fillers. Lower melt viscosity also improves the dispersion of the filler in the melt. The roundness of the fly ash particle further results in reduced abrasiveness compared to materials such as ground glass or wollastinite.
Accordingly, fly ash has been used as a filler at low loadings (1% to 2%) since the 1950""s to improve the stiffness of some plastic. However, certain disadvantageous properties of fly ash have limited its widespread usage. Untreated fly ash contains a broad range of particle sizes and, compared to more commonly used fillers, is very coarse. As an example, a high quality ground calcite based mineral filler would typically have a median particle size (D50) of from 2 to 5 xcexc. The presence of significant amounts of coarse material greatly contributes to the loss of strength of the filled polymer. Also, untreated fly ash contains carbon char particles and xe2x80x9cmagnetitexe2x80x9d particles (a highly substituted iron-rich spinel) which are undesirable as they adversely affect color, increase the bulk density and may cause problems with polymer crystallization.
Beneficiation of fly ash is required in order to produce a quality mineral filler. Contaminants must be removed, and the finest particle sizes separated efficiently. It is known in the art to accomplish such beneficiation of fly ash for use as, e.g., an admix in concrete, by such means as hydraulic classification and flotation separation. Accordingly, consideration was given to simply extending the severity of the hydraulic classification step as described in U.S. Pat. No. 5,817,230 to Groppo et al., incorporated herein by reference and describing a method for producing an improved pozzolan. Briefly, this technology employs hydraulic classification to remove the coarse ash as well as dense magnetite particles. A second screen may be employed to recover very light materials and/or plant debris if it is encountered, as in the case for fly ash recovered from a landfill or sediment pond. This step is then followed by froth flotation to recover the carbon and reduce the loss on ignition to acceptable levels.
However, merely extending the severity of hydraulic classification was found unsuitable. Two different phenomena had to be overcome to beneficiate the fly ash sufficiently to produce a suitable filler grade fly ash material for plastics, concrete, and the like. First, the problem of hindered or xe2x80x9cblanketxe2x80x9d settling had to be addressed. Blanket settling refers to the phenomena whereby in a mix with differing particle sizes all of the particles settle concomitantly due to larger particles entraining smaller particles and hindering their movement. Thus no differentiation based upon particle diameter is possible. Blanket settling is generally known to occur in particulate materials with high pulp densities and large differences in particle sizes such as untreated fly ash.
It is also known that fly ash tends to flocculate in water, thereby further reducing the ability to differentiate based on particle size. The flocs formed are remarkably stable even at elevated pH values. It is believed that the surface of the siliceous particles of fly ash have a less strong or dense surface charge compared to naturally occurring minerals. It is known that generally, natural flocculation can only occur if the surface charges of the particles are at or near the point of zero charge (PZC).
Accordingly, there is need in the art for a method of treatment of fly ash for removal of contaminants and efficient separation of the finest particle sizes to result in a suitable filler material for, e.g., plastic polymers, concrete, and cement. To have practical economic value, the recovery efficiency of the small particles (i.e. yield) must be relatively high. The end product must also have a very small median particle size with a narrow size range distribution, with minimal contamination by larger particle sizes. Finally, the technology must operate at reasonable feed pulp density to generate product at a reasonable rate to allow the use of generally available conventional equipment.
In accordance with the purposes of the present invention as described herein, a process for producing fly ash filler material with a uniform mean particle size in the range of 2-4 xcexcm is provided. The fly ash provided by the method of this invention is suitable as a filler material for, e.g., plastic polymers, mortar, and concrete. In one aspect, the method of the present invention comprises the step of slurrying fly ash in water in the presence of a superplasticizer, followed by elutriating the resulting slurry by any suitable means such as an elutriation column or a hydraulic classifier. Suitable superplasticizers may be selected from the group including, but not limited to, sulphonated naphthalene-formaldehyde condensate, sulphonated melamine-formaldehyde condensate, polycarboxylates and any mixture thereof The superplasticizer of choice may be added to the slurry at a concentration of from about 1.0 g/kg of fly ash to about 8.0 g/kg of fly ash. In a preferred embodiment of the present invention, the superplasticizer is added to the slurry to near saturation, but preferably maintained below the saturation point.
In another aspect, the method of the present invention is conducted at a pH range of from about 7.5 to about 10.5. In one preferred embodiment of this invention, the method is conducted utilizing fresh fly ash at a pH range of from about 8.0 to about 9.5. In a particularly preferred embodiment, the method is conducted utilizing fresh fly ash at a pH range of from about 8.0 to about 8.9. In another preferred embodiment, the method is conducted on stored fly ash at a pH range of from about 9.5 to about 10.5. The method of this invention provides for adjusting the pH of the fly ash/water slurry to a desired range as described above.
In still another aspect of this invention, a composition for use in mortar and concrete is provided wherein fly ash filler material (super pozzolan) having a mean particle size in the range of 2-4 xcexcm is added in a sufficient amount to replace 20-30% of the added cement.
In yet still another aspect of this invention, a plastic polymer composition is provided wherein 20-30% of any suitable plastic polymer is replaced by fly ash filler material having a mean particle size of 2-3 xcexcm.
Other objects of the present invention will become apparent to those skilled in this art from the following description wherein there is shown and described a preferred embodiment of this invention, simply by way of illustration of the modes currently best suited to carry out the invention. As it will be realized, the invention is capable of other different embodiments and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.